Traditionally, when using a light source having an output intensity that varies greatly in spatial distribution (such as with a metal halide lamp, a xenon lamp or a halogen lamp), a method using lens arrays (e.g., lenticular lens arrays arranged in series) has been used in order to eliminate the spatial irregularity in intensity of the emitted light. This method is hereinafter called the xe2x80x9cintegratorxe2x80x9d method. Many other methods are also known in order to obtain a more even illumination, such as a light dividing technique as disclosed in Japanese Laid Open Patent Application H3-111806.
In an example of the integrator method, a prior art illuminating optical device employs, in the light path following a multiple-light-source optical illuminating device, a first lenticular lens array, a second lenticular lens array, and a field lens, respectively, in the order that the light transits each. Both lenticular lens arrays serve as a light integrator section and are constructed of multiple lens element regions arranged in a two-dimensional array. The light of a single light source, having a large spatial irregularity of luminous intensity, is projected along the optical axis of a concave mirror by being reflected at the surface of the concave mirror. The first lenticular lens array divides this single beam of light into multiple light beams, the number of which equals the number of the lens element surfaces in the first lenticular lens array. Thus, the irregularity of luminous intensity of each divided beam of the light beam will be smaller than the irregularity of luminous intensity of the single light beam before the division. Each divided light beam is then projected toward a different region of the second lenticular lens array. The second lenticular lens array and a field lens operate to direct each divided light beam so that all divided light beams overlap one another at an illuminated area, thereby achieving an even illumination in the illuminated area.
The second lenticular lens array serves as a secondary light source by positioning a projection lens so as to receive light transmitted by the second lenticular lens array. The first lenticular lens array serves as the input surface of the integrator section and the second lenticular lens array serves as the output surface of the integrator section. The multiple lens regions of the first lenticular lens array and the aperture of a light valve, such as an LCD array that is controllable by electrical signals, are arranged so as to be at conjugate points of an optical system. Thus, a projection-type display unit, such as a liquid crystal projector, can be implemented.
As described above, a projection-type display unit using two integrator surfaces that employ multiple light sources has been previously disclosed in Japanese Laid Open Patent Application H6-265887. In this publication, the light beams from the multiple light sources are parallel, or nearly parallel, with one another before being incident on the first surface of the integrator. Although this publication discloses using multiple light sources to obtain a bright image, the first integrator surface must necessarily be as large as the array of multiple light sources, and thus the over-all illuminating optical system is large and inefficient.
FIG. 12 shows an example of a prior art illuminating optical system that has multiple light sources. This illuminating optical system includes the light source device 910 and the integrator section 911. The light source device 910 is comprised of the multiple light sources 910A and 910B, which are each constructed using a lamp 904 and a reflector 901. The light sources 910A and 910B radiate light beams that are parallel, or nearly parallel, to the optical axis X of the integrator section. The integrator section 911 is constructed of a first integrator surface 911A and second integrator surface 911B. The size of the second integrator surface 911B is smaller than the size of the first integrator surface 911A. The first integrator surface 911A, which receives the light beams from the light sources 910A and 910B, must have a surface area at least as large as the sum of the surface areas of each parallel light beam. In other words, the first integrator surface must be large and have an overall shape so as to receive the output light from the array of multiple light sources. Otherwise, light will be wasted (i.e., not form part of the output light of the integrator section).
The present invention is an illuminating optical system formed of multiple light sources and an integrator section. The integrator section of the present invention can be more compact than in the prior art, resulting in increasing the illumination efficiency, and enabling not only the lenticular lens arrays that form the integrator section to be produced at less cost but also allowing a reduction in size and cost of a polarization converter and a field lens that are commonly used with the illuminating optical system.
In the present invention, rather than the multiple light sources radiating light parallel to the optical axis of the integrator section, as in the prior art, the multiple light sources are arranged so that their output beams at least partially overlap one another on the input surface of the integrator section. This enables not only the input surface area of the integrator section to be reduced by a factor depending on the amount of overlapping, but also enables the diameter of the field lens, as well as focal length of the lens regions on the first lenticular surface to be decreased by the same factor. Since the second lenticular lens array is positioned in the focal plane of the first lenticular lens array, this enables the distance between the first and second lenticular lens arrays to be decreased by the same factor while maintaining a constant FNO of the illuminating optical system. Thus, the illuminating optical system can be more compact in all three dimensions, and both a polarization converter as well as a field lens that are commonly used with the illuminating optical system can be reduced in diameter by the same factor. This results in significant cost savings in producing these components.